The present invention relates to analog circuits, and more particularly, to envelope detectors.
The Home Phoneline Networking Alliance (HomePNA) is an incorporated, non-profit association of companies working to bring networking technology to the home. See www.homepna.org. HomePNA envisions bringing Ethernet technology to the home by utilizing existing home phone wiring for the network physical medium. HomePNA provides specifications for the physical layer (PHY), its interface to an Ethernet MAC (Media Access Control), and its interface to the home phone wiring. See the IEEE (Institute of Electrical and Electronic Engineers) 802.3 standard for Ethernet.
The position of a HomePNA PHY in relationship to the OSI (Open Systems Interconnection) model is illustrated in FIG. 1. Logical Link Control (LLC) 102 and MAC 104 are implemented in accordance with IEEE 802.3, and HomePNA PHY 106 communicates with MAC 104 via interface 108. Additional sublayers, and other optional layers, may be added to the layers shown in FIG. 1 so that PHY 106 may provide services to other communication protocols, such as Gigabit Ethernet. In practice, PHY 106 and MAC 104 may be integrated on a single die, so that interface 108 is not readily visible.
PHY 106 receives a MAC frame from MAC 104, strips off the 8 octets of preamble and delimiter from the MAC frame, adds a HomePNA PHY header to form a HomePNA PHY frame, and transmits the PHY frame on physical medium 108. A PHY frame is transmitted on physical medium 108 utilizing pulse position modulation (PPM). All PHY symbols transmitted on physical medium 108 comprise a pulse formed of an integer number of cycles of a square wave that has been filtered with a bandpass filter. The position of the pulse conveys the transmitted symbol. Differential signaling is employed, in which a pulse and its negative are transmitted on two wires for each transmitted symbol.
FIG. 2 illustrates an example of received waveforms. Although differential signaling is employed, for simplicity FIG. 2 shows only a single-ended version of the received differential signal. Each symbol is 129 tics long, where 1 tic is defined as (7/60)10xe2x88x926 seconds, which is approximately 116.667 nanoseconds. Pulses are positioned within one of four time slots to convey two bits of information. The time slots are separated by 20 tics, and are at positions 66, 86, 106, and 126 tics from the beginning of a symbol interval. A special SYNC symbol, indicated as AID (Access Identification) symbol 0 in FIG. 2, is composed of a SYNC_START pulse beginning at tic=0 and a SYNC_END pulse beginning at tic=126. In FIG. 2, AID symbol 1 comprises a pulse in position 1 (tic=86), and AID symbol 2 comprises a pulse at position 2 (tic=106). A receiving PHY performs full-wave rectification of the received signals, and compares the envelope of the rectified signals with an AID slice threshold. The PHY detects a received pulse if its envelope exceeds the AID slice threshold. As soon as a pulse is detected by a PHY, the PHY disables further indications of detection until a time AID_END_BLANK (located at tic=61) from the beginning of the pulse, after which detection indication must be re-enabled for the next received pulse.
One approach for detecting the envelope of a differential signal is to form the difference of the received differential signals to provide a single-ended signal, followed by detecting the envelope of the single-ended signal. One such envelope detector is shown in FIG. 3. A single-ended voltage signal V is applied to non-inverting port 302 of differential amplifier 304 and to inverting port 306 of differential amplifier 308. Diodes 310 and 312 provide full-wave rectification, so that capacitor 314 charges when either diode 310 or diode 312 is forward biased. The output voltage Vout at node 316 is proportional to the envelope of the input signal V. Resistor 318 allows capacitor 314 to discharge when there is no received signal.
The present invention improves upon the envelope detector of FIG. 3 for networks employing differential signaling.